Economic inequality. It is both highly moralized (right-thinking people agree it’s the root of all evil) and intellectually devilishly complex, far more than people acknowledge. For example, if “the bottom fifth” earns the same proportion of income in 1980 and 2010, it doesn’t mean anyone’s income stagnated: these “fifths” are different people, and they earn a fifth of different totals. Also, it’s not clear that inequality (as opposed to poverty) is a moral abomination, or that reducing it is progress. As Walter Scheidel argues in The Great Leveler (another superb book of 2017), the most effective ways of reducing inequality are epidemics, massive wars, violent revolutions and state collapse.